Bandpass radomes constructed with Frequency Selective Surfaces (FSS) typically use resonant FSS elements that are approximately one half of a wavelength long in their largest dimension at the passband center frequency. Such half-wavelength elements typically exhibit multiple resonances such that, at normal incidence, a radome having a passband centered at fo exhibits spurious resonances at 3fo, 5fo, 5fo, etc. At oblique incidence, spurious resonances may also occur near 2fo, 4fo, 6fo, etc. In addition, such resonant element radomes will typically support the propagation of undesired surface wave electromagnetic wave modes excited at edges of the structure or at other discontinuities. The surface waves can radiate energy to produce radiation pattern anomalies for an antenna system where the radome is used to cover the antenna.
Bandpass radomes may be used in antenna system applications where one desires to allow transmission of electromagnetic waves in one or more ranges of radio frequencies and to suppress the transmission of waves at other frequencies. Such bandpass radomes may have dielectric layers that are each approximately λ/4 (one-quarter of a free-space wavelength) in thickness. At high microwave frequencies, λ/4 is a relatively small dimension, but at UHF frequencies (300 MHz to 1 GHz) or even low microwave frequencies (1-3 GHz), λ/4 can be too large for some applications. Hence there exists a need for electrically-thin and physically thin bandpass radomes. Furthermore, thin bandpass radomes may have less mass than conventional bandpass radomes due to thinner dielectric layers.